[GIS] Distance between points and polygons in QGIS / PostGIS

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I need to work out the distance between a table of polygons and a table of points.

The table of polygons has different site boundaries, and the point table has points relating to each of the site boundaries.

I need to work out the distance between each point and the site it relates to.

What is the best way to do this? Is this best done with a PostGIS query or is there a way to do it in QGIS (I notice there is distance matrix tool in QGIS but it doesn't support polygons). Ideally I would like a way to do this in QGIS as its something my users do quite regularly.

Best Answer

I would create a view in the database with the distance calculated.

Then you can open that view in QGIS and get your distances. When the tables changes the view will be updated.

/Nicklas

Related Solutions

[GIS] Calculate min distance between points in PostGIS

It's not a simple problem, it involves some kind of forced iteration over the set of candidate points. This chapter from the workshop shows a similar problem, but not exact (your problem is slightly easier)

http://postgis.net/workshops/postgis-intro/advanced_geometry_construction.html

The nearest neighbor searching chapter from the workshop shows the tools you might use to do an index-assisted approach with some external loop driving the query

http://postgis.net/workshops/postgis-intro/knn.html

If your points have a distinct id and you know a distance tolerance (9999) they will all fall within, a self-join and use of the "DISTINCT ON" filter will get you the answer in one go.

WITH unfiltered AS
(
  SELECT t1.id AS id1, t2.id AS id2, ST_Distance(t1.geom, t2.geom) as dist
  FROM t t1, t t2 WHERE ST_DWithin(t1.geom, t2.geom, 9999) AND t1.id <> t2.id
  ORDER BY t1.id, ST_Distance(t1.geom, t2.geom) ASC
)
SELECT DISTINCT ON (id1) id1, id2, dist FROM unfiltered;

It first gathers the candidates combinations of points, and sorts them by distance. Then the "distinct on" filter strips out just the first member of each candidate group, which conveniently is the closest, thanks to the pre-sorting.

[GIS] QGIS: calculate distance between points

Have you tried the Points2One plugin mentioned in this post.

You should be able to create lines, grouped by the point IDs. Once you have the line layer you could calculate a line length to get the distance between the two points.

FYI - using the plugin on Mac created the line layer but wouldn't add it to my project, I thought it did't work but it does. You just have to load your line layer back into the project.

Best Answer

Related Solutions

[GIS] Calculate min distance between points in PostGIS

[GIS] QGIS: calculate distance between points

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